Mobile telephone networks typically include a mobile switching center MSC and a number of registers in the form of databases accessed during the establishment of a telephone call connection or other events in the network. These databases include a home location register HLR and a visitor location register VLR. They store both static (i.e., non-changing) and dynamic (i.e., changing) data related to the subscribers. The static data includes, for example, a list of identifiers of cooperating exchanges interconnected with the mobile switching center, an identification of the services permitted for each subscriber, and parameters for such services. The dynamic data can include, for example, subscriber defined service data such as preferences, mobile station location data, and mobile station activity data.
An HLR database has to be extremely robust. The failure of an HLR normally brings a GSM/UMTS network to a halt. As the HLR gets progressively larger with improvements in technology, even partial failures of the database become a major concern for the operator. Each partial failure may impact a large number of subscribers resulting in huge losses of revenue. This invention eliminates outages caused by such failures in a seamless fashion. One example of such failures is geographical disasters. Whilst it is a common expectation that the HLR is completely destroyed, it is not impossible to find disasters where there is partial loss of equipment.
The HLR contains data needed to establish a telephone connection within the mobile telephone network, so any failure of the HLR will deprive a substantial number of subscribers of service, and correspondingly affect operator revenue. Conventionally, the hardware and software is arranged for high reliability and availability, typically using mirroring techniques and local backup storage. To give some geographical redundancy, each HLR is often coupled to another HLR at a different location, to create a mated pair and the load is split between them, often but not necessarily, in 50—50 proportion. If one has all or the majority of the load, it is referred to as the master HLR, and the other as the slave. The two HLRs are located with sufficient geographical separation to provide resilience to local events such as floods or earthquakes. In the case of a disaster affecting one of them, they are designed to have sufficient capacity for one to handle the entire load. This requires a cutover operation. A GSM/GPRS/UMTS (Global system mobile)/(General Packet Radio Service)/(Universal Mobile Telecommunication System) HLR Mated-Pair Disaster Cutover (also referred to as failover) involves an HLR in a mated-pair arrangement detecting that it's mate HLR has undergone a “disaster” and then “switching over” to provide an active service for the subscribers belonging to the mate HLR (as well as continuing to provide an active service for its set of home subscribers). Note that the word “disaster” in this context includes one of the HLRs in a mated-pair becomes inaccessible (Total Route Failure) i.e. all network communication is lost to one of the HLRs, and one of the HLRs in a mated-pair goes out of service (Nodal Failure) e.g. caused as a result of a natural phenomena (e.g. Earthquake).
U.S. Pat. No. 5,623,532 discloses a system where two HLRs support each other to provide geographical redundancy, via an SS7 (Signaling System No. 7) telecommunications network without the need for additional links or interface modules between the two mated HLRs. The two HLRs, are connected through the same two Signal Transfer Points (STPs). Each node in a SS7 telecommunications network is supported by dual STPs. In case the first STP or links between the first STP and the destination node fails, the second STP is utilized to provide reliable network operation by passing the messages for the failed HLR to its paired HLR. Determination of failure of an HLR is made manually by an administrator, or by the STPs, not by the paired HLR.
U.S. Pat. No. 5,953,662 also shows having two HLRs located anywhere within the SS7 network and supporting each other in real time without requiring additional communications links between the two and without destroying the integrity of the data base. This patent goes further than the '532 patent in that it shows the HLRs sending messages to each other over the SS7 network. One use for such messages is for a first HLR to update the contents of its data base to conform to that of its paired HLR so that it can take over at any time from the paired HLR, and vice versa. The actual transmission is achieved over the same SS7 telecommunications network utilizing the same Signal Transfer Points (STPs).
The HLRs also monitor each other for failure by sending occasional heartbeat messages to each other. A lack of response to a heartbeat is interpreted by a first HLR as indicating a failure of the other HLR. As the lack of response lasts longer, the perceived failure status of the paired HLR is upgraded from temporarily out of contact to inoperable. As before, should the other HLR fail, signals from other entities intended for the other HLR are rerouted by the local STP of the SS7 network to the first HLR: for processing.
The problem of recovering from faults and the resumption of service of subscribers belonging to such GSM/UMTS HLR databases undergoing failures increases as the size of the databases increases. Very large databases now imply a size typically above several million subscribers. A typical size would be 10 million subscribers. It is known to provide redundancy within one HLR by using servers with active back up at the same location. In addition, geographical disasters would be dealt with by having redundancy in the form of mated-pairs. Commonly, the implementation for disaster recovery is a complete takeover of the faulty HLR and repair of the HLR followed by a re-mating process. The short coming of this is that it is expensive to takeover and recover a mated-pair relationship of a complete HLR. Data can be lost in the takeover. Recovery for a very large HLR takes a lot of effort and is thus costly.